Saving hybrid pressure to netcdf

This PR adds the ability for Iris to save cubes with hybrid pressure aux factories (i.e. a  ```atmosphere_hybrid_sigma_pressure_coordinate``` dimensionless vertical coordinate) to netcdf. In the process it also add the ability to save cubes with multiple aux factories to netcdf. It also fixes a bug where previously the ```coord.name()``` was used in the formula terms rather than the cf variable names.